Uptake of Organic Contaminants from Soil into Vegetables and Fruits - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 391

9.6.1 Crop Types and Uptake Pathways
The crop type has a very large impact on uptake processes, e.g. roots and potatoes
are in close contact to soil, while apples are not. As a consequence, the accumulation
of contaminants from soil is much lower in apples and other tree fruits, whereas
the accumulation by uptake directly from air is higher for fruits. It is obvious that
crop-specific models will give more realistic predictions of concentrations.
Even between different species of the same crop type differences in uptake can
exist. For zucchini and pumpkin ( Cucurbita pepo ), both members of the plant family
Cucurbitaceae and the genus Cucurbita , root uptake and subsequent translocation
to shoots and fruits was the main uptake route of PCDD/F, probably due to root exu-
dates which mobilize lipophilic contaminants (Hülster and Marschner 1995 ) and an
increased mobility in the transpiration stream (Gent et al. 2007 ). However, fruits
and leaves from other plant species, even for the closely related cucumber plant
( Cucumis sativus ), were mainly contaminated by airborne PCDD/F (Hülster and
Marschner 1994 ; Hülster et al. 1994 ). The exact reason for the ability of cucurbita
plants to extract lipophilic contaminants from soil and transport themwith the xylem
sap is yet unknown, but it was observed repeatedly, and for a number of lipophilic
organic contaminants, such as p,p -DDE (White 2002 ), DDT (Lunney et al. 2004 ),
PCB (Whitfield Åslund et al. 2007 ) and PBDE (Mueller et al. 2006 ). For phenan-
threne, unusual high adsorption to a range of plant species was observed (Zhu et al.
2007 ).
9.6.2 Physiological Parameters
The importance of physiological plant-specific parameters, such as transpiration
rate, leaf area, conductance, water and lipid contents as well as growth rate, depends
largely on the properties of the contaminant. For water soluble contaminants, which
are rapidly translocated from soil to leaves, the transpiration rate is among the most
important parameters, since the accumulation in leaves is almost directly dependent
on the transpiration.
This is illustrated with the example of carbofuran in Fig. 9.9 .Thelog K OW of this
contaminant is 1.6-2.07 (Trapp and Pussemier 1991 ). Two calculations were done,
one with the standard transpiration of 1 L d − 1 , the other with an increased transpira-
tion of 5 L d − 1 . The simulated concentration in roots remains nearly constant with
increasing transpiration, only the time period until steady state is reached is shorter.
However, the breakthrough of contaminant into leaves is faster and the calculated
concentration in leaves is a factor of 5 higher for the increased transpiration rate
due to their direct relationship. The translocation of carbofuran in bean plants was
determined experimentally, and the concentration pattern as well as the close rela-
tion between transpiration and concentration in leaves was confirmed (Trapp and
Pussemier 1991 ).
In turn, the amount of transpired water depends on temperature, humidity, leaf
area and stomata permeability. However, the transpiration depends also on the